A flocculating-mixing tank of the related art is, for example, described in Japanese Patent No. 4761216 (Patent Literature 1). In the flocculating-mixing tank, stirring blades are vertically arranged in two stages and an upward flow of slurry containing a flocculant is supplied to be stirred and mixed. The flocculating-mixing tank is divided into a lower rapid stirring unit and an upper slow stirring unit. The rapid stirring unit has a body inside diameter smaller than that of the slow stirring unit.
The rapid stirring unit has a rapid stirring blade and a drive shaft that drives the rapid stirring blade. The rapid stirring blade includes one turbine blade that has a plurality of wings radially disposed on a surface of a rotating plate with the drive shaft located at the center.
The slow stirring unit includes a slow stirring blade and a drive shaft that drives the slow stirring blade. The slow stirring blade includes a flat paddle blade.
In the rapid stirring unit, a flocculant is dispersed in water to be treated to form nucleus for flocculated flocs, and then solids in the treated water are flocculated around the nucleus to form fine floc. At this point, without rapidly and sufficiently dispersing the flocculant, fine flocs are insufficiently formed so as to inhibit the formation of flocculated flocs during a slow stir, leading to degradation of dewatering of a dewatering machine, a thickener, and the like in the subsequent steps.